The subject matter of the present invention relates generally to amplitude sorting of oscillatory burst signals by converting such burst signals into output pulses whose height is proportional to the maximum amplitude of the burst signals. More specifically, the invention involves detecting the oscillatory burst signals to provide corresponding burst envelope signals, sampling of the midportion of the envelope to produce an output sample pulse whose height is proportional to the amplitude of the envelope signal and the corresponding maximum amplitude of the burst signal, and transmitting the sample pulses to a pulse height analyzer for sorting.
The method and apparatus of the present invention are especially useful in an acoustic emission testing system for amplitude sorting of the acoustic emission signals in order to identify the flaws or other material anomalies producing such signals in the material being tested. However, the present invention is also useful in amplitude sorting of any oscillatory burst signals where the rise times of the burst signal envelopes vary in an unpredictable manner, because it employs delayed sampling to sample the midportion of such envelopes.
Previous acoustic emission testing apparatus, such as that described by P.H. Hutton in the article, "Acoustic Emission Applied Outside Of The Laboratory", in Acoustic Emission, published by the American Society for Testing and Materials, special technical bulletin 505, 1972, pages 114-128, do not identify the flaws satisfactorily. Thus, while conventional acoustic emission testing systems have the ability to detect and locate the origin of acoustic signals emitted from the flaws within the test material, they have great difficulty in identifying the types of flaws and distinguishing between insignificant and significant flaws. These problems are overcome by the present invention. The prior testing apparatus does not convert the acoustic emission burst signals into output pulses whose height is proportional to the amplitude of the burst signal envelope so that such output pulses may be transmitted to a pulse height analyzer for amplitude sorting in the manner of the present invention. By sorting and counting the acoustic emission burst signals according to their amplitude, it is possible with the invention to identify the material anomalies, such as porosity, cracks, or other flaws, emitting such acoustic signals in the test material with a much greater degree of accuracy.
The present invention sorts oscillatory burst signals according to their amplitude by detecting such signals to produce a corresponding burst envelope signal and sampling an intermediate portion of such envelope signal occurring after its leading edge has risen completely, to produce a sample output pulse whose height is proportional to the amplitude of the envelope signal and the corresponding maximum amplitude portion of the oscillatory burst signal. These sample output pulses are then fed to a pulse height analyzer for counting and sorting according to their pulse height, into groups of pulses, which groups correspond to certain material anomalies or flaws.
It is known to sample the midportion of a signal by delayed triggering of a sampling pulse generator, as shown in U.S. Pat. No. 3,470,482 of G. R. Kolnowski, grated Sept. 30, 1969. However, there is no teaching in this patent of producing an envelope signal of an oscillatory burst signal and sampling such envelope signal to produce sample pulses of corresponding height which enable amplitude sorting of such burst signals in the manner of the present invention. In addition, this prior sampling apparatus has the disadvantage that the sampling pulse generator can be prematurely triggered by reflections or other transient signals in the input signal. This is avoided in the present circuit by providing a lockout multivibrator which prevents retriggering of the sampling pulse generator for a predetermined adjustable time. Thus, signal reflections of a given oscillatory burst signal in the acoustic emission test material are prevented from retriggering the sampling pulse generator.